In thermal printheads, a plurality of heating dots are arranged linearly with a predetermined pitch. The dots are selectively actuated in accordance with printing data for performing printing. The printing may be performed by a heat transfer method using ink ribbon or by a more direct method using thermosensitive recording paper. The printing is performed for one line at a time. The printing speed is increased as the printing cycle for one line is shortened. Accordingly, the feeding speed of the recording paper is increased. Recently, an effort has been made to provide a thermal printhead having thus increased printing speed to meet the requirement of a higher-speed printer.
In general, heating dots of a thermal printhead tend to accumulate heat because of the presence of a glaze layer. When the printing speed is increased, such a heat-accumulating tendency of the heating dots will give rise to the following problem.
It is now assumed that each of the heating dots is simply heated equally in accordance with the printing data. In case where the printing is performed to make printing dots consecutive in the secondary printing direction (which is the direction in which the recording paper is fed relative to the head), a drive energy is repetitively given to the heating dot which has been already heated by the immediately preceding drive of the heating dot for the primary scanning (in the direction of the array of heating dots) before the temperature of the heating dot drops, resulting in the increased heat accumulation at the heating dot. As a result, even if a printing operation for the next line is not expected according to the printing data, in other words, even if the consecutive driving operations of the heating dot are completed, the so-called trailing phenomenon will occur in which printing is performed on the recording paper due to the accumulated heat.
Various methods of controlling driving of a thermal printhead have been proposed to avoid the above problem, even in performing high-speed printing.
Typical examples are disclosed in Japanese Patent Application Laid-open Nos. 61-116555 and 4-305471 for instance. According to these, the printing energy to be given to the currently selected printing dot is controlled by referring to the previous ones of the printing data for the selected heating dot and for the adjacent heating dots, or in other words by referring to the history information about whether printing was performed by these dots for the last printing line and the printing line before last, or further for more previous printing lines. With such a controlling method in general, the energy to be given to the currently selected heating dot is reduced as the number of times printing was previously performed by the selected heating dot and the adjacent heating dots, for the purpose of reducing a possibility of the above-mentioned trailing phenomenon.
However, even when the above controlling method capable of reducing the trailing phenomenon is adopted, it is still difficult to completely avoid the trailing phenomenon when printing is performed at much higher speeds. Further, the following problem will occur. Specifically, the trailing phenomenon is such an occurance that printing is erroneously performed in a non-printing region near the boundary between the printing region and the non-printing region as viewed in the secondary scanning direction. However, the following problem due to the heat accumulated in the selected heating dot will occur across the boundary from the non-printing region to the printing region.
Specifically, even if a certain printing energy is given to the selected heating dot to print the first line of a printing region that is adjacent to the last line of the previous non-printing region, the heating dot may fail to be sufficiently heated up due to the shortened printing cycle for much faster printing performance. As a result, it is possible that the printing dot supposed to be printed for the first line is not formed on the recording paper. Unless such a problem taking place at the boundary from a non-printing region to a printing region is overcome, serious inconvenience as follows will result even if the trailing phenomenon can be avoided. Specifically, in an instance where high-speed printing is performed to print a bar code in the direction perpendicular to the bars, a relatively narrow bar may not be printed at all or be reduced in width more than a predetermined value, so that the specific information the bar code is supposed to represent as a whole may be altered in its meaning.
In order to prevent such a problem taking place at the boundary from the non-printing region to the printing region, it may be possible that a relatively larger amount of energy is given to the selected heating dot for printing the first line of the printing region. However, according to such a method, it is necessary to prepare a power source capable of giving such increased printing energy to all of the arrayed heating dots. This is very disadvantageous in terms of the cost.